An optical head device is employed for writing an optical information to or reading an optical information from an optical recording medium such as an optical disk or an optical magnetic disk. In such an optical head device, emission light from a laser diode as a light source, is converged by an objective lens on a recording plane of a disk-shaped optical recording medium (hereinafter referred to as “optical disk”), to carry out writing and/or reading of an information. At a time of e.g. reading an information, emission light reflected by an information recording plane of an optical disk is received by a photodetector.
By the way, with respect to read-out of information, the shorter the wavelength of laser beam in use is, the more the recording density can be increased, and thus, use of emission light of shorter wavelength (405 nm) from a light source, for an optical head device, has been in progress in recent years. Meanwhile, such an optical head device is, at the same time, required to be capable of reading with laser beam of longer wavelengths (660 nm and 790 nm) for many optical disks that have been widely used. For this purpose, e.g. JP-A-2004-158118 proposes various types of optical head devices using laser beams of conventional longer wavelengths and laser beams of shorter wavelengths to realize compatibility of optical disks.
With respect to writing of information, in order to obtain this compatibility with conventional optical disks, a type of optical head device has been considered, which employs light sources of longer wavelengths in addition to a light source of shorter wavelength for high-density recording. Meanwhile, in order to realize an optical head device capable of handling high-density optical disks and writable type optical disks such as DVD-Rs or CD-Rs, high light-utilization rate is required for each wavelength for each type of optical disk.
For this reason, a polarization type optical head device has been considered, which employs a polarizing diffraction element having high transmittances for outgoing path and high diffraction efficiency for returning path. Here, “outgoing path” means a direction of emission light from a light source to an optical disk and “returning path” means a direction of emission light reflected by an information recording plane of an optical disk towards a photodetector, namely, a direction in which reflected and returned light propagates.
FIG. 14 shows an example of the construction of an optical head device having a polarizing optical system employing conventional three different laser beams, namely, three types of laser beams.
In FIG. 14, linearly polarized laser beams from a laser diode 101A emitting wavelength of 405 nm, a laser diode 101B of 660 nm and a laser diode 101C of 790 nm, are transmitted through a polarizing hologram 102A for 405 nm, a polarizing hologram 102B for 660 nm and a polarizing hologram 102C for 790 nm, respectively, that have high transmittance for linearly polarized incident light. Then, these linearly polarized laser beams are transformed into circularly polarized laser beams by a quarter wavelength plate 103A for 405 nm, a quarter wavelength plate 103B for 660 nm and a quarter wavelength plate 103C for 790 nm, respectively, that are each integrally formed with a polarizing hologram.
Thereafter, the laser beams are transformed into parallel beams by a collimator lens 104A, a collimator lens 104B and a collimator lens 104C that are disposed separately, transmitted through or reflected by a beam splitter 105 having a characteristic of transmitting 405 nm and reflecting 660 nm, and further, transmitted through or reflected by a beam splitter 106 having a characteristic of transmitting 405 nm and 660 nm and reflecting 790 nm. Then, these laser beams are converged on an information recording plane (hereinafter simply referred to as “surface of optical disk”) of an optical disk D by an objective lens 108 common to three wavelengths, which is held by an actuator 107.
Further, the reflected light beams from an optical disk D containing information of pits formed on a surface of the optical disk, propagate inversely through the respective paths. Namely, circularly polarized light beams whose rotation directions are inverted by reflection at the surface of the optical disk D, are transmitted again through the quarter wavelength plate 103A, the quarter wavelength plate 103B and the quarter wavelength plate 103C, respectively, to be transformed is into linearly polarized light beams having polarization directions perpendicular to the respective incident polarization directions, and diffracted by the polarizing hologram 102A, the polarizing hologram 102B and the polarizing hologram 102C to be diffracted light beams. Information of pits of the optical disk D included in these diffracted light beams, are detected by a photodiode 109A as a photodetector for 405 nm, a photodiode 109B for 660 nm and a photodiode 109C for 709 nm, to read out information recorded on the surface of the optical disk D.
In a conventional optical head device using a plurality of wavelength regions such as 405 nm, 660 nm and 790 nm, it is proposed to use common optical elements such as a quarter wavelength plate (for example, refer to JP-A-10-68816). However, a phase plate (quarter wavelength plate) for converting linearly polarized light beams of two wavelengths such as wavelengths 405 nm and 660 nm, to circularly polarized light beams, cannot convert a linearly polarized light beam of wavelength 790 nm to a complete circularly polarized light beam. In the same manner, a phase plate for converting linearly polarized light beams of wavelengths 660 nm and 790 nm to circularly polarized light beams, cannot convert a linearly polarized light beam of 405 nm to a complete circularly polarized light beam, and thus, desired characteristics cannot be obtained.
Further, JP-A-2002-156528 describes that a broadband phase plate can be constituted by a single phase plate without laminating phase plates. The broadband phase plate is designed so that its retardation value reduces as the wavelength value becomes shorter. However, in order to obtain a function of complete quarter wavelength plate in entire region of wavelength from 400 to 780 nm by using the single broadband phase plate, design of the material is extremely difficult and such a broadband phase plate is not satisfactory for high-recording-density use which requires high-light-utilization efficiency.
Further, hereinafter, in an optical head device to be used for an optical recording medium for which higher light-utilization efficiency is required such as a Blu-Ray Disk that is expected to be a next-generation standard which uses a light source of further shorter wavelength, optical elements such as the above-mentioned quarter wavelength plate do not have sufficient characteristics. For example, in an optical head device using three wavelengths 405 nm, 660 nm and 790 nm, it is necessary to dispose total three sets of optical elements that are optical elements for 405 nm, optical elements for 660 nm and optical element for 790 nm, which causes problems that the number of components is increased to increase the volume of the device and that it takes longer time for assembly and adjustment.
Meanwhile, in order to downsize an optical head device, it is proposed to dispose two laser diodes closely or to employ a laser diode capable of emitting a plurality of wavelengths. However, in these cases, it is difficult to switch optical paths for respective wavelengths even by using a beam splitter having different reflectivities at different wavelengths. Thus, it is desired to use optical elements common to these wavelengths.